It is known in the art to use heat treatable aluminium alloys in a number of applications involving relatively high strength, high toughness and corrosion resistance such as aircraft fuselages, vehicular members and other applications. Aluminium alloys AA7050 and AA7150 exhibit high strength in T6-type tempers. Also precipitation-hardened AA7×75, AA7×55 alloy products exhibit high strength values in the T6 temper. The T6 temper is known to enhance the strength of the alloy, wherein the aforementioned AA7×50, AA7×75 and AA7×55 alloy products which contain high amounts of zinc, copper and magnesium are known for their high strength-to-weight ratios and, therefore, find application in particular in the aerospace industry. However, these applications result in exposure to a wide variety of climatic conditions necessitating careful control of working and ageing conditions to provide adequate strength and resistance to corrosion, including both stress corrosion and exfoliation.
In order to enhance resistance against stress corrosion and exfoliation as well as fracture toughness it is known to artificially over-age these AA7000-series alloys. When artificially aged to a T79, T76, T74 or T73-type temper their resistance to stress corrosion, exfoliation corrosion and fracture toughness improve in the order stated (T73 being best and T79 being close to T6) but at the cost of strength compared to the T6 temper condition. A more acceptable temper condition is the T74-type temper which is a limited over-aged condition, between T73 and T76, in order to obtain an acceptable level of tensile strength, stress corrosion resistance, exfoliation corrosion resistance and fracture toughness. Such T74 temper is performed by over-ageing the aluminium alloy product at temperatures of 121° C. for 6 to 24 hours and followed by 171° C. for about 14 hours.
Depending on the design criteria for a particular aircraft component even small improvements in strength, toughness or corrosion resistance result in weight savings, which translate amongst others to fuel economy over the life time of the aircraft. To meet these demands several other 7000-series alloys have been developed.
For example each of EP-0377779, U.S. Pat. No. 5,221,377 and U.S. Pat. No. 5,496,426 disclose alloy products and an improved process for producing an 7055 alloy for sheet or thin plate applications in the field of aerospace such as upper-wing members with high toughness and good corrosion properties which comprises the steps of working a body having a composition consisting of, about in wt.%: Zn 7.6 to 8.4, Cu 2.2 to 2.6, Mg 1.8 to 2.1 or 2.2, and one or more elements selected from Zr, Mn V and Hf, the total of the elements not exceeding 0.6 wt. %, the balance aluminium plus incidental impurities, solution heat treating and quenching the product and artificially ageing the product by either heating the product three times in a row to one or more temperatures from 79° C. to 163° C. or heating such product first to one or more temperatures from 79° C. to 141° C. for two hours or more and heating the product to one or more temperatures from 148° C. to 174° C. These products are reported to have an improved exfoliation corrosion resistance of “EB” or better with about 15% greater yield strength than similar sized 7×50 counter-parts in the T76-temper condition. They still have at least about 5% higher strength than their similar-sized 7×50-T77 counterpart (7150-T77 will be used herein below as a reference alloy).